Not Applicable.
Not Applicable.
The present invention relates to a filtration membrane and a method of making this membrane. More specifically, this invention relates to a hydrophilic hollow fiber membrane that is formed from a hydrophobic polymer.
Membranes are thin-film barriers that allow certain components of a fluid mixture to selectively pass through the barriers while discriminating against the other components to achieve separation. These membranes are typically formed from polymers and are semipermeable. The specific physical shape or form of the membranes can vary, and can include flat sheets, tubular membranes, and hollow fibers. The specific use to which the membrane is to be put dictates the form selected for its construction. Membranes in the form of hollow fibers are currently used in a variety of applications, including dialysis, gas separation, ultrafiltration, microfiltration, and nanofiltration.
Polyvinylidene fluoride (PVDF) based membranes have good mechanical strength and excellent chemical stability, particularly to free chlorine attack. Unfortunately, membranes made of PVDF homopolymer are hydrophobic, and water cannot wet the surface of a hydrophobic PVDF membrane in the absence of a surfactant. Thus, the hydrophobic nature of PVDF membranes imposes an enormous resistance to water permeation to give a low water flux. In addition, hydrophobic PVDF membranes often suffer from a severe fouling problem due to non-selective absorption of solutes at the hydrophobic membrane surface to further lower permeate flux.
In order to improve the hydrophilicity of PVDF membranes and to reduce membrane fouling, chemical surface modification has been used to prepare hydrophilic PVDF based membranes. One method of modifying a PVDF membrane is by first reducing the PVDF membrane with NaOH and NaS2O4, followed by oxidizing it with NaOCl, creating a more hydrophilic membrane.
An alternative method of chemical surface modification that has been proposed involves using calcined alumina in particle form to replace NaOH to catalyze an elimination reaction of hydrofluoric acid (HF) from the PVDF backbone to give a double bond. A subsequent modification reaction is then completed by reaction either with water or with a partially hydrolyzed polyvinyl acetate so as to form a hydrophilic membrane.
Still another method of modifying the chemical surface of a PVDF polymer involves reacting PVDF powder with KOH in methanol, followed by reacting it with 98% H2SO4 to give a hydrophilic hydroxyl-containing PVDF membrane. This modified membrane has less fouling than before modification.
It has also been suggested to graft an epoxide-containing polymer to a PVDF membrane in order to improve membrane mechanical strength and hydrophilicity. Still further, grafting a polymer containing a positively charged organic phosphonium compound onto the PVDF membrane surface so as to make it more hydrophilic has also been proposed. In addition, it has been proposed to covalently bond quaternary ammonium groups to positively charged PVDF membranes. Such a membrane has been used for pharmaceutical separations. Still further, others have suggested a process for preparing hydrophilic microporous PVDF membranes by grafting a water-soluble polymer, such as polyethylene glycol dimethacrylate, to the hydrophobic membrane substrate surface by irradiation means, such as ultraviolet irradiation.
While chemical modification permanently adds hydrophilic groups to the PVDF membrane by covalent bonding, the membranes created by such modification have disadvantages. The modification reaction often has a low yield and poor reproducibility. In addition, many times toxic chemicals are used in the modification reaction. Still further, the process may be lengthy and costly.
An alternative approach to improving the hydrophilicity of PVDF membranes is to blend a hydrophilic polymer with hydrophobic PVDF. Components that can be blended with PVDF include cellulose acetate, sulfonated polysulfone, glycerol monoacetate, glycerol diacetate, glycerol triacetate, and sulfonated polyetherketone.
The polymer blend approach has a lower cost and higher efficiency than chemical modification. However, the polymer blend approach has some drawbacks. Because there is no covalent bonding between the PVDF and the hydrophilic components, it is often found that membrane performance deteriorates with time due to a gradual loss of hydrophilic components from the membrane matrix.
Another method that has been suggested is surface coating. For instance, a hydrophobic PVDF membrane may be coated with a water-insoluble vinyl alcohol-vinyl acetate copolymer. The coating layer however is more vulnerable to free chlorine attack than PVDF. Therefore, after frequent exposure to a cleaning reagent containing free chlorine, such as bleach, the hydrophilic coated membrane becomes hydrophobic.
A water-soluble polymer, such as polyvinylpyrrolidone (PVP), has not been used as a part of a polymer blend to make a hydrophilic PVDF membrane because the water-soluble polymer is washed out of the membrane by water, as is taught by U.S. Pat. No. 5,151,193. U.S. Pat. No. 5,834,107 (the ""107 patent) contradicts this teaching but is technically inaccurate. If the PVDF membrane disclosed in the ""107 patent contained 1-30% by weight polyvinylpyrrolidone, as claimed, then it would be hydrophilic, as represented in the ""107 patent. However, the membrane of the ""107 patent actually is not hydrophilic and does not contain 1-30% by weight polyvinylpyrrolidone. Evidencing the fact that this membrane is not hydrophilic, the ""107 patent teaches that its membrane must be exposed to a wetting agent, such as hydroxypropylcellulose, in order to make it hydrophilic. This would not be necessary if the membrane was really hydrophilic. What actually happened in the making of the membrane of the ""107 patent was that PVP was added into the membrane casting solution as a pore former and then washed out of the membrane by water in a coagulation bath during membrane formation. In fact, such a process is discussed in U.S. Pat. Nos. 5,151,193 and 4,399,035, where PVP is used as an additive to fabricate a PVDF membrane.
Still further, in the ""107 patent, the membrane was cast in an environment having a relative humidity as high as 100% at 27xc2x0 C., but water vapor pressure was not increased by increasing temperature. Instead, this patent discloses using a longer exposure time of the cast membrane to humid air. This is disadvantageous because longer exposure times of the cast membrane to humid air prohibits membrane production at a higher speed. For instance, if the exposure time is 2 minutes, as suggested in the ""107 patent, and the membrane casting speed is 10 ft/min, it requires 20 feet of exposure space. Thus, the process of the ""107 patent requires a huge capital investment to make a membrane casting machine to meet the requirement of a two minute exposure time. Furthermore, gravity will cause the extruded fiber to break before it reaches the coagulation bath if the distance between the spinneret and the coagulation bath is too long.
In order to overcome the deficiencies found with the membranes discussed above, a membrane with strength and hydrophilicity and a process for making the same are needed. More specifically, a membrane whose hydrophilic qualities are not washed away with water or bleach is needed. Still further, a process for making such a membrane that is efficient, has a good yield, and is easily reproducible is needed. In addition, a better way to control the humidity of the environment where the membrane is created is needed in order to improve the productivity of membrane manufacturing.
It is an object of the present invention to provide a hydrophilic membrane that has the mechanical strength and chemical stability of a PVDF membrane and a method of making this membrane.
It is another object of the present invention to control humidity during membrane formation so as to improve membrane structure and performance.
According to the present invention, the foregoing and other objects are achieved by a hydrophilic membrane that includes a hydrophobic polymer and a water-soluble polymer-metal complex. This membrane is made by heating a mixture of a hydrophobic polymer, an additive, and a solvent, adding a metal compound and a water-soluble polymer to the mixture, and heating and mixing the solution. The water-soluble polymer forms complexes with the metal compound and homogeneously entangles with the dissolved hydrophobic polymer to form a viscous dope. The dope is extruded through an annular orifice to form a hollow fiber. The fiber is put in an environment having a controlled humidity so that it becomes partially solidified, and then, the fiber is put in a coagulation bath. The hollow fiber is formed by phase inversion in the coagulation bath and is collected with a take-up wheel partially immersed in a leaching bath.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.